Engineered Electroactive Microbes as Building Blocks for Living Electronics

Electroactive bacteria, such as <i>Shewanella oneidensis</i> and <i>Geobacter sulfurreducens</i>, can couple the oxidation of organic electron donors to the reduction of external solid surfaces, including minerals and electrodes. To carry charge from within the cell to external surfaces, these electroactive bacteria utilize outer membrane multiheme cytochromes. These multiheme cytochromes can (1) facilitate long-distance (micrometer-scale) redox conduction along the outer membrane and (2) reduce metal ions in solution for the biogenic synthesis of technologically relevant nanomaterials. By exerting control over these bioelectronic properties with synthetic biology and electrochemistry, the foundation can be laid for developing living electronics, devices that combined the properties of both biology and solid-state electronics. To this end, we implemented an optogenetic biofilm patterning gene circuit and a small molecule sensor in <i>S. oneidensis</i> to control cytochrome expression in response to added concentrations of vanillic acid. This allowed us to pattern cells on electrode surfaces with light and to tune the electrochemical activity, conduction, and the intrinsic conductivity of living biofilms as a function of cytochrome expression. Additionally, using <i>G. sulfurreducens</i>, we allowed a biofilm to form on electrodes ahead of Pd nanoparticle biomineralization for biofilm-localized synthesis of biogenic nanomaterials. This allowed us to create a hybrid nanoparticle-cell film. Here, we have used different combinations of synthetic biology, electromicrobiology, and biomineralization to develop methods for controlling biofilm geometry, conductivity, and nanomaterial synthesis. Thus, just as photolithography, ion implantation, and metal deposition form the building blocks of solid-state device fabrication, perhaps similar capabilities for patterning, tuning electrical properties, and material synthesis demonstrated here in electroactive microbes can become the building blocks for the fabrication of living electronics.